Enhanced, durable silver coating stacks for highly reflective mirrors

ABSTRACT

The disclosure is directed to a highly reflective multiband mirror that is reflective in the VIS-NIR-SWIR-MWIR-LWIR bands, the mirror being a complete thin film stack that consists of a plurality of layers on a selected substrate. In order from substrate to the final layer, the mirror consists of (a) substrate, (b) barrier layer, (c) first interface layer, (d) a reflective layer, (e) a second interface layer, (f) tuning layer(s) and (g) a protective layer. In some embodiments the tuning layer and the protective layer are combined into a single layer using a single coating material. The multiband mirror is more durable than existing mirrors on light weight metal substrates, for example 6061-Al, designed for similar applications. In each of the five layer types methods and materials are used to process each layer so as to achieve the desired layer characteristics, which aid to enhancing the durability performance of the stack.

This application is a continuation of U.S. patent application Ser. No.15/265,941, filed on Sep. 15, 2016, which is a continuation of U.S.patent application Ser. No. 13/834,230, filed Mar. 15, 2013, whichclaims the benefit of priority under 35 U.S.C. § 119 of U.S. ProvisionalApplication Ser. No. 61/770,548 filed on Feb. 28, 2013, the contents ofwhich are relied upon and incorporated herein by reference in theirentirety.

FIELD

The disclosure is directed to Enhanced, Durable InnovativeSilver-containing (“EDIS”) coating stacks for highly reflective mirrors,and to a method of making such stacks, for use in ISR detectors.

BACKGROUND

There have been constant improvements inIntelligence-Surveillance-Reconnaissance (ISR) detector technology, forexample, increasing the wavelength detection range, decreasing detectorfootprint and reducing pixel size, all of which enable the system sizeand weight to be reduced. These improvements created an increased demandfor ISR broad-multi-band optical systems; specifically including a highdefinition visible band (VIS, 0.34 μm to ˜0.75 μm) as well as the nearinfrared (NIR, 0.75 μm to 1.4 μm), the short wavelength infrared (SWIR,1.4 μm to 3 μm), the mid-wavelength infrared (MWIR, 3 μm to 8 μm), andlong wavelength infrared (LWIR, 8 μm to 15 μm) bands. A key component ofsuch systems is reflective optics that have a silver coating thereonwhich enables the systems to achieve this spectral performance. However,historically the silver coatings have been a source of system failuresdue to the propensity of the silver coating to “break-down” or “corrode”over time. It may not be possible to completely protect future systemsfrom some of the harsh environments in which they will operate,particularly environments that are hot, humid and contain salt. It isthus desirable to have a system having highly durable broad band silvercoated optics.

There are several test procedures that are used to evaluate thedurability performance of thin film coated optical components. Examplesinclude military specification documents such as MIL-C-48497,MIL-F-48616 and MIL-PRF-13830B, which include tests that involveexposure to humidity, salt fog, salt solutions, temperature cycling,abrasion, and other test procedures others. The harshest of these testsis the “24-hour salt fog” test. While at the present time there are twogroups that claim highly reflective mirror stacks, made by specificprocesses, that can pass the 24-hour salt fog test (Quantum Coating Inc.using a Denton Vacuum process (not described) and Lawrence LivermoreLaboratories using a process described in U.S. Pat. No. 7,838,134),neither of these stacks meet the entire ISR spectral requirements,specifically they do not meet them for the LWIR range. TheQuantum/Denton silver coating is called “X-1 Silver” by both parties;and the published information shows performance in the 0.4 μm to 0.7 μmrange (2000 Society of Vacuum, Coaters 505/856-7188; 43^(rd) AnnualTechnical Conference Proceedings (2000) ISSN 0737-5921), with noinformation for longer wavelengths into the infrared out to the LWIRrange being given for this coating. This article also suggests the useof ion beam assisted deposition and substrate heating might furtherimprove performance. However, substrate heating is not desirable whenusing some metallic substrates, for example 6061-Al substrates, becauseif the temperature is too high the mechanical strength and corrosionresistance of the substrate is decreased. Consequently, it is preferredthat the substrate temperature be below the heat treating (˜415° C.) andstress relief (˜350° C.) temperatures of the 6061-Al substrates.Lawrence Livermore Laboratories, U.S. Pat. No. 7,838,134, claims the useof nitrides for the silver adhesion-interface layers while using a Si₃N₄protective cap layer. The patent states that the “silver mirror ischaracterized by high reflectance in a broad spectral range of 300 nm inthe UV to the far infrared (˜10000 nm)”, this range being 0.3 μm to 10μm.

However, despite the advances made in the art, further extension of thesilver mirror's reflectance properties, including the wavelength rangeinto the LWIR and mirror durability, is desirable.

SUMMARY

The disclosure is directed to a highly reflective multiband mirror thatis reflective in the VIS-NIR-SWIR-MWIR-LWIR bands, the mirror being acomplete thin film stack that consists of six (6) types of layers on asubstrate. The multiband mirror is more durable than existing mirrors onlight weight metal substrates, for example 6061-Al, designed for similarapplications. In each of the layer types methods and materials are usedto process each layer so as to achieve the desired layercharacteristics, which aid to enhancing the durability performance ofthe stack. While any given layer may improve stack durability, it is thecombination of these five layer types that results in the higher,enhanced level of performance exhibited by the mirror of thisdisclosure. Table 1 illustrates these 5 types of layers, which are abarrier layer, at least one interface layer, a reflective layer, atuning layer and a protective layer, and defines their properties.Multiple materials can be used to meet the characteristics needed foreach of the different layer types.

TABLE 1 Coating Design and Materials Layer Layer CharacteristicsExemplary Materials Protective The layer possesses a high level of bothchemical YbF₃, YF₃, Si₃N₄, Layer(s) and mechanical durabilityYb_(x)F_(y)O_(z) Tuning A low refractive index, high refractive indexYbF₃, YF₃, GdF₃, Layer(s) design is used for layer tuning. The materialsBi₂O₃Yb_(x)F_(y)O_(z), must be low absorbing in the wavelength rangeBi₂O₃, ZnS of from 0.4 μm to 15.0 μm, and possess a medium to high levelof chemical and mechanical durability. Second This layer is used topromote adhesion of the Bi₂O₃, ZnS, Nb₂O₅, Interface tuning layers tothe reflective layer, must have TiO₂, Ta₂O₅, Al₂O₃ Layer low absorptionand be galvanically compatible (2^(nd) layer) with the reflective layerReflective Silver (Ag) is used as the reflective layer to Ag, Au, Al,Rh, Cu, Layer provide high reflectance (high % R) in the Pt, Niwavelength range of layer possesses a high level of both chemical andmechanical durability First Interface This interface layer is used topromote adhesion Al₂O₃, TiO₂, Bi₂O₃ Layer between the barrier layer andthe reflectance ZnS, Ni, Bi, Monel (1st Layer) layer, and must havegalvanic compatibility with (Ni—Cu), Ti, Pt the barrier and reflectivelayers. Barrier Layer This layer is used to create galvanic Si₃N₄, SiO₂,TiAlN, compatibility between the Ag reflective layer TiAlSiN, TiO₂, DLC,(0.15 V) and the Al substrate (0.9 V). While Al Al, CrN,Si_(x)N_(y)O_(z) and CrN can be barrier layers, an interface layer isneeded to achieve galvanic compatibility with the reflective layer.Substrate A light weight diamond turned optic/substrate, silica, fusedsilica and for example 6061 aluminum (6061-Al) and other F-doped fused;and Al alloys, Mg alloys, Ti Alloys and ceramic 6061-Al alloy, otheralloys. A barrier layer is needed for metal alloys light weight Alalloys and its thickness is substrate dependent. Mg alloys, Ti alloys 1.The interface layers are also referred to as “adhesion” layers. 2.Yb_(x)F_(y)O_(z) is formed when oxygen is admitted during thedepositions of YbF₃. Electronically, nYb⁺³ = yF⁻¹ + zO⁻² so that the sumof the positive and the negative valances balance and there is no netcharge to the coating layer. The same is true for Si_(x)N_(y)O_(z).Thus, in order from substrate to the final layer, the mirror consists of(a) substrate, (b) barrier layer, (c) first interface layer, (d)reflective layer, (e) second interface layer, (f) tuning layer and (g)protective layer. In some embodiments, the tuning layer and theprotective are combined into a single layer using a single coatingmaterial.

Barrier Layer

The thickness of the barrier layer can be in the range of 100 nm to 50μm. In one embodiment, the barrier layer has a thickness in the range of500 nm to 10 μm. In another embodiment, the barrier layer has athickness in the range of 1 μm to 5 μm. One criterion for determiningthe thickness of the barrier is the number of hours the article willhave to withstand the salt fog test. The longer the duration of the saltfog test the thicker the barrier layer. For a salt fog test of 24 hoursa barrier layer of 10 μm has been found sufficient. In manyapplications, if the barrier layer is too thick it will cause distortionof the finished part with changes in temperature, but since typicallythe operational temperature is given in the specification the thicknessof the barrier layer can be adjusted to prevent distortion. Thedifferences in the thermal expansion coefficients of the barrier layerand the substrate will cause the optical figure, power and irregularity,to change ΔT (the change in temperature). In some embodiments, thebarrier layer is sufficiently thick so that it will cover or smooth outany high and irregular substrate peak-to-valley variations. Smoothingout such variations aids in polishing the surface to optimize surfacequality. The surface quality is important in promoting adhesion onentire surface and minimizing localized defect sites that may be causedby the peak-to-valley variations.

The First and Second Interface Layers, Also Known as “Adhesion Layers”

The thickness of these layers is dependent of factors including thematerial used for the layers, whether the layer is the 1^(st) or 2^(nd)interface or adhesion layer, and whether it is on the front surface (thesurface of the reflecting layer) or back surface (the layer on which thereflecting layer is deposited) of the mirror. When Ni, Cr and Timaterials are used as the interface layer only a thin layer of material,on the order of angstroms “Å”, is used. For a front surface mirror theinterface layer on top of the reflecting layer, that is, the secondinterface layer, needs to be thick enough to promote adhesion, but alsothin enough so that it does not absorb any of the reflected radiation.In general, the thickness of the first interface layer is in the rangeof 2 Å to 250 Å (0.2 nm to 25 nm). For metallic interface layers, forexample Ni and Cr, the thickness is in the range of 2 Å to <25 Å (0.2 nmto <2.5 nm). In one embodiment the metallic first interface layerthickness is in the range of 2 Å to 10 Å (0.2 nm to 1 nm). When a metaloxide or sulfide, for example Al₂O₃ or ZnS, is used as the firstinterface layer the thickness is greater than 25 Å (>2.5 nm). In anembodiment the first interface layer is a metal oxide or sulfide thethickness is in the range of 50 Å to 250 Å (5 nm to 25 nm). In anotherembodiment the first interface layer thickness is in the range of 10 nmto 20 nm.

The interface-adhesion layer used under the reflective layer, that is,the first interface layer, is present only to promote adhesion so thatoptical considerations such as absorbing reflection radiation is not aconsideration. Consequently, the thickness of the first interface oradhesion layer is determined based on adhesion and not opticalconsiderations. As a result, the first interface layer can have aminimum thickness to provide the adhesion, but no maximum thicknessbecause there are no absorption or optical concerns. However, thethickness of the second interface layer has to be carefully controlledso the reflection losses are minimized. For the second interface layerthe thickness is in the range of 5 nm to 20 nm. In an embodiment thethickness of the second interface layer is in the range of 8 nm to 15nm. In another embodiment where reflective article is intended for usein the wavelength range of 9.5 μm to 15 μm, the thickness of the secondinterface layer is in the range of 8 μm to 12 μm to maximize thereflectance of the final reflective article.

Reflective Layer

For the reflective metal layer the thickness must be sufficient toprovide optimum reflection properties. If the reflective layer is toothin the film is not continuous and/or transmitting and if it is toothick this can create durability concerns. The thickness of thereflective layer is in the range of 75 nm to 350 nm. In an embodimentthe thickness of the reflective layer is in the range of 80 nm to 150nm. In another embodiment the thickness of the reflective layer is inthe range of 90 nm to 120 nm.

In addition to using Ag as the reflective material, the stackconfiguration can also be used with other reflective materials, forexample Au, Al, Rh, Cu, Pt, Ni, to provide an enhanced durable, chemicaland mechanical performance, but with a change in spectral range andreflection intensity.

Tuning and Protective Layers:

The thicknesses of these layers depend on the optimization necessary toachieve the required spectral performance while simultaneouslyoptimizing the protection necessary to pass the required tests, forexample, the salt fog and humidity tests. The thickness of these layerscan vary significantly depending on application and materials used.

An Ag mirror prepared using the scheme and materials described in Table1 will have the following advantages 1-7.

-   -   1. It will meet a high reflectivity specification in all desired        wavelength bands, VIS-NIR-SWIR-MWIR-LWIR, for angle on incidence        (AOI) of 0 to 45 degrees and in some cases >45 degrees. The        materials and thicknesses used to prepare an Ag mirror as        described herein has resulted in minimum absorption in LWIR        region where other stacks and prior art have absorption issues.        The materials used in the present disclosure include YbF₃, YF3,        ZnS, Bi₂O₃, GdF₃ that have no absorption bands in the defined        LWIR band, 8 μm to 15 μm.    -   2. YbF₃ and YF₃ have demonstrated high resistance to the salt        fog environment and pass specification moderate abrasion tests,        so they are alternative materials that can be used as a        protective cap layer    -   3. The Al₂O₃, Bi₂O₃, ZnS and TiO₂ transparent materials, used as        an interface-adhesion layer, can be thicker on the front end        interface; thus it is easier to control layer termination during        the deposition process compared to ultra-thin layers of Cr or        Ni, or related alloys. These materials, Al₂O₃, ZnS and TiO₂,        have considerably less absorption in all bands compared to the        metals typically used for this purpose, for example, Ni and Cr.    -   4. Replacing a metal and/or conductive interface-adhesion layer        with non-conductive materials results in ideal galvanic        compatibility.    -   5. Replacing a metal and/or conductive barrier layer with        dielectric material results in ideal galvanic compatibility.    -   6. Thick barrier layers have increased overall stack resistance        to salt fog and extended humidity environments. Materials        successfully used in achieving this resistance are Si₃N₄, SiO₂,        Si_(x)N_(y)O_(z), DLC (diamond-like carbon), TiO₂, CrN,        Si_(x)N_(y)O_(z), TiAlN or TiAlSiN or similar composite films.        In addition, if needed to optimize galvanic compatibility, an        optional multi-layer barrier could be configured and applied as        a non-conducting layer at the metal/substrate surface with the        addition of a thicker conductive layer, for example, SiO₂—CrN.        For example, CrN is a very good barrier layer because of its        resistance to alkali and (possibly) low stress, as well as ease        of deposition process (control and deposition rate), but the        galvanic compatibility of CrN with the Al or metal alloy is not        optimum because it is conductive. Using a thinner dielectric        layer like SiO₂ or Si₃N₄, that has high intrinsic stress, will        isolate the CrN from the metal substrate, thus creating optimum        galvanic compatibility. In addition, one can also design and use        a combination of different materials to create a “barrier stack”        so as to have minimum stress. For example, a highly compressive        stress film layer stacked with a highly tensile stress film        layer will cancel each other resulting on zero or minimal        stress.    -   7. A thick Al layer on top of the 6061-Al diamond turned        substrate with a thin barrier layer increased the overall stack        resistance to salt fog and extended humidity environments. In        addition, one can consider the Al layer deposited on top of the        6061-Al substrate as being a barrier layer in its own right or        as the first later of a barrier stack. In addition, these thick        layers can be polished prior to the deposition of additional        layers. The polishing will result in improved surface quality        that cannot be achieved with the bare Al-6061 substrate. The use        of a thick SiO₂ layer for this purpose is known (U.S. Pat. No.        6,921,177).        As mentioned above, mirrors prepared using other reflective        metals will have similar advantages though with changes in        spectral range and reflection intensity.

The barrier layer, the first and the second interface layers, the silverlayer and the tuning layer can each, independently, be deposited usingion assistance. In some embodiments, ion assistance is mot used or isused for only part of the deposition process. In all embodiments, theprotective layer is deposited using ion assistance. However, thesematerials can also be deposited using other processes and will performwell over the 0.34 μm to 15 μm wavelength range, with the provision thatthe process will optimized for the characteristics desire. Ion assistunder the correct conditions optimizes stoichiometry and density andpossibly structure. An example, different techniques were used toprepare a mirror having the 0.34 μm to 15 μm wavelength rangecharacteristics described herein. The techniques selected were dependenton the materials being used and the goal that was to be achieved.

TABLE 2 Exemplary Materials and Techniques Material Process Comments DLCRF-PECVD process Thick Al Ultra-high rate thermal deposition orsputtering. No IAD. Substrate source distance critically close. TopAl₂O₃ (a) No Initial IAD, then deposit with IAD after first 2-4 nm; or(b) No IAD at all YbF₃ or YbF_(x)O_(y) IAD with or without the use of O₂Nb₂O₅ E-beam IAD or U.S. Pat. No. 5,851,365 Si₃N₄ or SiO₂ E-beam IAD orU.S. Pat. No. 5,851,365

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a graph illustrating the effect of ion-assisted deposition(IAD) of the Ag layer on the performance of a substrate/CrN/Ag mirrorboth before and after a 2 hour salt fog (SF) test compared to a mirrorin which IAD was not used.

FIG. 1B is an optical microscope image (magnification 374×) illustratingthe surface deterioration, the dark spots indicated by the arrows 18,the after 2 hours exposure to salt fog of a mirror in which the silvercoating was deposited without IAD assistance.

FIG. 1C is an optical microscope image (magnification 374×) illustratingthe surface deterioration, the dark spots indicated by (arrows 18, after2 hours exposure to salt fog of a mirror in which the silver coating wasdeposited with IAD assistance, the deterioration being retarded relativeto that in FIG. 1B as a result of using ion assistance during thedeposition of the silver layer.

FIG. 2A a graph of Reflectance versus Wavelength of Ag/Cr/Si₃N₄ andAg/Ni/Si₃N₄ mirrors after 0, 4, 6 or 10 hours of salt fog (SF) exposure;numerals 20 and 22 being the Cr-containing mirror after 0 and 4 hours SFexposure, respectively, and numerals 24, 26 and 28 being theNi-containing mirror after 0, 6 and 10 hours SF exposure.

FIG. 2B is a photograph of mirror illustrating the effects of galvanicpotential difference on controlled stack of coating on a glasssubstrate, the stacks being Ag/Cr/Si₃N₄ (upper photograph) andAg/Ni/Si₃N₄ (lower photograph) after 4 hours and 5 hours salt fogexposure, respectively.

FIG. 3 is a graph illustrating the AL-O absorption band at approximately10.7 μm for an Al₂O₃ greater than 100 nm thick.

FIG. 4 is a graph of the absorption band for Si—N and Al—O atapproximately 9.1 μm and 10.7 μm, respectively, for Si₃N₄ and Al₂O₃ thinfilms that are less than 100 nm thick.

FIG. 5A is a graph of a partial stack illustrating the reduction inreflectivity when a thin protective of Si₃N₄ is placed on top of anAl₂O₃ second interface layer covering the Ag layer, the graph showing areduction in reflectivity not only from 0.5 μm to 0.8 μm, but also outto 1.6 μm. This is not due to absorption part (k) of the dispersionproperties but to refractive index n and matching these indices in thestack design. FIG. 5B is a graph illustrating a theoretical stack andshowing how adjusting the materials affects the reflectivity.

FIG. 5B, instead of having just Si₃N₄ top of an Al₂O₃ second interfacelayer as in FIG. 5A, has a three part coating consisting ofSi₃N₄—SiO₂—Si₃N₄ placed on top of the Al₂O₃ layer. This illustrates thatthe protective layer has to be designed to not only protect the coatingand make it more durable, but also to optimize or enhance thereflectivity of the desired wavelength bands.

FIG. 6A is a graph illustrating the absorption band for Nb₂O₅ and YF₃when the coating thickness of these materials is >100 nm.

FIGS. 6B and 6C are graphs illustrating the optical performance of anactual stack designed for VIS-SWIR-MWIR-LWIR band performance, withadjustment to the Nb₂O₅ layer to minimize absorption in the LWIR band.

FIG. 7 is a graph of Reflectance versus wavelength illustrating that theuse of a high refractive index material, for example YF₃, in the desiredMWIR and LWIR bands which results in a higher % R performance.

FIG. 8A is provides graphs and an illustration of a mirror, fused silicasubstrate, with an EDIS coating having a barrier layer after greaterthan 23 hours exposure to salt fog showing that there was no measurablechange in performance after salt fog exposure; the configuration being,from substrate to top layer, fused silica, Si₃N₄ or CrN, Al₂O₃, Ag,Al₂O₃, YbF₃, Nb₂O₅, YbF₃.

FIG. 8B provides a graph and an illustration of a mirror, 6061-Alsubstrate, with an EDIS coating and a thick barrier layer after greaterthan 23 hours exposure to salt fog; the graph illustrating that there isa potential for causing out of specification Δ-figure and Δ-temperatureperformance; the configuration being, from substrate to top layer,6061-Al, ultra thick Ni barrier, Al₂O₃, Ag, Al₂O₃, YbF₃, Nb₂O₅, YbF₃. Noblemishes were observed on the mirror after the salt fog testing. The Niwas deposited by electroless plating on the 6061-l substrate. Regardingthe performance of a coated mirror with regard to temperature changes,this is particularly related to the CTE of both the substrate andbarrier layer. The difference between the two will cause distortion tothe optic or a change in figure with changes in temperature. The thickerthe barrier layer the more of a change in figure, and the largerdifference in the CTE of the substrate and barrier layer the more theincrease in figure change. Consequently, it is necessary to design-inthe CTE and the thickness in order to minimize Δ-figure andΔ-temperature.

FIG. 8C is an illustration of a mirror similar to that of in FIG. 8Bexcept that the mirror has a thin barrier layer after 6 hours salt fogtesting; the mirror showing a blemish on the left at the arrow.

FIG. 8D is a complete stack formed on turned 6061 aluminum that passedall tests after exposure to 120 hours of humidity exposure (3.4.1.2),moderate abrasion (3,4,1,3 and adhesion testing (3.4.1.1).

FIG. 9 are interferometer results showing precipitate peaks or noduleson the surface of diamond turned and polished 6061-Al before coating;the being characterized using EDS and identified as being impurities inthe 6061-Al alloy. These large precipitates create a non-homogenoussurface which makes it difficult to obtain a surface finish less than 60A rms, the best results being about 30 A rms, which was obtained withdifficulty.

DETAILED DESCRIPTION

Herein the term “high reflectance” means a reflectance of at least areflectivity of at least 95% over the wavelength range of 0.4 μm to 15μm. Also herein the phrase “salt fog” is abbreviated as “SF”. The6061-Al aluminum substrate, or other metallic substrates, is diamondturned and polished before the application of any coating materials.Glass, glass-ceramic or ceramic substrates are ground, lapped andpolished before the application of any coating. The abbreviation “AOI”means Angle of Incidence” and is in degrees, and the term “pol” means a“polished aluminum substrate.” In the process described herein it isdesirable that the substrate temperature be minimized during thedeposition of the coating materials. When 6061-Al substrates are usedthe temperature should be below the heat treating and stress relieftemperatures of 6061-Al which are 415° C. and 350° C., respectively.

The Reflective Layer:

Due to the multiband reflection requirements, ranging from visible (VIS)through the long wave infrared (LWIR), 0.40 μm out through 15 μm, a thinfilm layer of silver is used for the reflective layer. Silver is knownto have the highest reflectivity, lowest polarization splitting, andlowest emissivity through this entire wavelength range. [See S. A.Kovalenko and M. P. Lisita, “Thickness dependences of optical constantsfor thin layers of some metals and semiconductors,” SemiconductorPhysics, Quantum Electronics and Optoelectronics Vol. 4, No. 4, pages352-357 (2001); Chang Kwon Hwangbo, et al, “Ion assisted deposition ofthermally evaporated Ag and Al films”, Applied Optics Vol. 28, No 14,(Jul. 15, 1989); and N. Thomas et al, “Protected Silver Coating forFlashlamp-Pumped Nd:glass Amplifiers,” 30th Annual Symposium on OpticalMaterials for High Power Lasers; Boulder, Colo. Sep. 30-Oct. 2, 1998;(preprint from Lawrence Livermore Laboratories; site locationhttp://library.llnl.gov/uhtbin/cgisirsi/mgYv2G09Sa/MAIN/103110005/60/502/X;search term “30^(th) annual symposium,” Paper #1, 236354(UCRL-JC-135179, preprint). The following characteristics are criticalto many multiband imaging systems.

-   -   (1) The silver layer must have a minimum thickness to obtain        optimum reflectivity. It is suggested in the literature that the        thickness be on the order of 150 nm, depending on the process        used to deposit the silver. A silver layer thickness in the        range of 135 nm to 175 nm is beneficial.    -   (2) The process that is used to deposit the silver layer        influences the durability of the silver layer.    -   (3) FIGS. 1A-1C illustrate how ion-assisted deposition (IAD) of        silver improves it chemical durability. It was noted that there        was some reflection loss is seen at the 400 nm range of the        pretest IAD scan, probably due to the trapping of gas atoms used        for bombardment.

In FIGS. 1A-1C the substrate was silica glass having a CrN barrier layeron top of the glass and a silver layer deposited on top the barrierlayer. No coating layers were applied on top of the silver layer.Numeral 10 designates an article where the silver layer was deposited ontop of the barrier layer without ion-assistance and the reflectancemeasured after deposition, but before salt fog testing. Numeral 12designates the article of numeral 10 after it has been exposed to saltfog for 2 hours. Numeral 14 represents an article where the silver wasdeposited on top of the barrier layer with ion-assistance and thereflectance measure after deposition, but before salt fog testing.Numeral 16 represents the article of numeral 14 after it has beenexposed to salt fog for 2 hours. The graph clearly indicates that thatwithout ion assistance the reflectivity of the silver coatingdeteriorates much more quickly then the coating with ion assisteddeposition after just 2 hours of salt fog testing. In FIG. 1B, taken at374× optical magnification, dark “spots” 18 indicated by arrows are thecorrosion that has taken place on the silver coating surface. In FIG. 1Cthere are much fewer corrosion spots and those that are present are muchsmaller which indicates the clear advantage of ion assistance during thedeposition of the silver reflecting layer.

Barrier Layer

Since Al-6061 is the substrate material used in these applications(though other light weight, diamond machined alloys, silica, fusedsilica and F-doped fused silica can also be used), a barrier layer mustbe used between the silver layer and the substrate, or an Al layerdeposited on any of the foregoing substrates to create galvaniccompatibility. The military standards for the use of dissimilar metalsare defined in MIL-STD-889B and MIL-STD-1250. These documents suggest,for systems that are expected to be exposed to harsh environments suchas hot and humid and/or containing salts, that dissimilar metals shouldnot be joined or interfaced if they exceed a galvanic potentialdifference of 0.25V (in a high humid environment with no salts thepotential difference can be >0.45V). In some of the engineeringliterature on corrosion a potential difference of 0.15V is suggested forharsh salt environments. Al-6061 is considered an anodic material with apotential of 0.90V while silver, a cathodic material, has a potential of0.15V, resulting in a potential difference of 0.75V. Interfacing anodicmetals to Al as the barrier material, for example cadmium, iron, andcarbon, results in a low potential difference of <0.25V. The galvanicpotential difference is >0.25V for tin, 0.33V for chromium, 0.33V forzinc, 0.63V for nickel 0.83V for magnesium. We have also effectivelyTiAlN (this TiAlN can be made to behave like metal or like a dielectricdepending on the Ti—Al ratio), and dielectric coatings such as diamondlike carbon (DLC), Al₂O₃, Si₃N₄, Si_(x)N_(y)O_(z), SiO₂, and TiO₂. WhileCrN has been used with some success, care must be exercised in view ofthe intended application because its galvanic compatibility isborder-line.

The surface quality of the 6061-Al also plays an important role. Largeprecipitate sites are formed by the “impurities” in the 6061-Al, some ofwhich come from the controlled addition of materials required in orderto meet material specifications for strength characteristics, and otherimpurities are simply contaminants. The large precipitates make itdifficult to achieve a smooth surface, <30 Å rms, and some of the highpeaks or nodules may result in poor adhesion (or cracking from stressesor voids) between the substrate and the coating stack (silver layer, orsilver layer covers differently), resulting in a defect site once thecompleted mirror is exposed to the environmental testing using both saltfog and long term humidity conditions. An approach to manage thiscondition is to deposit a very thick barrier layer that result ineffectively coating over these sites. FIG. 9, illustrating the finishedsurface of a 6061-Al substrate before is it coated with any material,shows the presence of these nodules. Barrier layer materials areselected from the group consisting of Si₃N₄, Si_(x)N_(y)O_(z), SiO₂,TiAlN, TiAlSiN, TiO₂, Si_(x)N_(y)O_(z) and DLC, and additionally Al orAl₂O₃.

The presence of large precipitates creates a non-homogenous surfacewhich makes it difficult to obtain a surface finish less than 60 Å rms,the best results being about 30 Å rms, which was obtained withdifficulty. The presence of the nodules serves to illustrate why thepresence of the barrier works to improve reflectivity. Without beingheld to any particular theory, the nodules can become defect sites wherelocalized corrosion occurs when exposed to these harsh environments.They may result in poor adhesion, so coating cracks or falls off atsites exposing areas or creating pathways. A sufficiently thick barrierlayer can smooth out this surface and create a continuous film with goodadhesion across the entire surface. If this barrier layer surface issufficiently thick, polishing the layer prior to the placement ofadditional coating layers would result in better surface finish in theapproximate range of 5 Å to 15 Å.

The thickness of the barrier coating can be in range of 10 nm to 100 μm.When the nodules or other surface defects are present on the substrateand cannot be removed, the barrier coating is in the higher end of therange and must be sufficient to cover the nodules. If the substrate issubstantially free of the nodules then the barrier coating can be at thelower end of the range. In addition, the use of ion assistance duringthe deposition of the barrier layer will densify the barrier coating andaid in providing a smooth surface.

Interface Layer A and B

Silver and gold have considerably lower oxide formation energiescompared to other metals like titanium, aluminum, chromium, and nickel,and because of this silver and gold do not adhere well to manymaterials. It has been known for some time that ultra-thin films of Crand Ni, or alloys of these metals, are excellent adhesion promotinglayers for silver due to the metal-to-metal diffusion with Ag (or gold),along with metallic bonding strengths between Ag or Au and Cr or Ni.Because of the environments the mirrors disclosed herein will be exposedto, galvanic compatibility is critical and therefore must be consideredwhen choosing the interface material. The galvanic potential differenceat the interface of silver-Cr and silver-Ni are 0.45V and 0.15Vrespectively. Table 1 illustrates the significant role the galvanicpotential difference has in the performance of the coating stack when itis exposed to a salt fog environment. Nickel or Al₂O₃ have been used asthe first interface layer between the barrier and the silver layersbecause the two materials are compatible.

Al₂O₃ has been discussed in the literature as an adhesion promotingmaterial for certain metals; specifically the discussion involvesAg—Al₂O₃ and Al—Al₂O₃ non-stoichiometric interfaces, and how theyinfluence adhesion (W. Zhang and J. R. Smith, Nonstoichiometricinterfaces and Al ₂ O ₃ adhesion with Al and Ag, Physical ReviewLetters, Vol 85, No 15, Oct. 9, 2000, pages 3225-3228; Jiwei Feng, etal., Ab initio study of Ag/Al ₂ O ₃ and Au/Al ₂ O ₃ interfaces, PhysicalReview B, 72, 115423, Sep. 21, 2005). The data in these papers showdeviations of Al₂O₃ from stoichiometry at the interface cansignificantly affect adhesion with the either Ag or Al metal, two metalschosen for their oxide heats of formation being at opposite end of therange. M. A. Scobey, U.S. Pat. No. 5,851,365 titled “Low PressureReactive Magnetron sputtering apparatus and method,” describes theconditions for two types of deposition processes: ion-assisteddeposition (IAD) and e-beam deposition, and a low pressure reactivemagnetron sputtering process, that produce optimum adhesion betweenAl₂O₃—Ag, and Al₂O₃—Al. Due to the first interface being on the backsideof the reflective layer, between the substrate and the Ag layer, it'supper limit of thickness is not limited by absorption, but should bemonitored for stress considerations. On the front side of the reflectivelayer, the second interface layer, the layer deposited on top of the Aglayer and the thickness of interface must be limited to minimize itsabsorption band in the LWIR band while obtaining optimum adhesion to Ag.This absorption band is illustrated in FIG. 3 which appears as the peakat approximately 10.7 μm. ZnS is an additional non-conducting materialthat can be used as an adhesion layer, avoiding galvanic compatibilityissues. For example, ZnS has been found to be a successful interfacematerial, for example, at a gold interface Au—ZnS—YbF₃ or Ag—ZnS—YbF₃.

The Protective Layer and Tuning Layer(s)

Silver can react with various substances that may be present in theatmosphere, for example salts, acids, and sulfur compounds. Well knownexamples are silver tarnishing which is the formation of black silversulfide (Ag₂S) by the reaction of Ag⁰ with sulfur containing compoundsand silver corrosion which results from the reaction of Ag⁰ withhalogen-containing substances in the atmosphere, the most common ofwhich is probably NaCl and HCl (T. E. Graedel, Corrosion Mechanisms forSilver Exposed to the Atmosphere, J. Electrochemical Society Vol. 139,No. 7, pages 1963-1969 (1992), and D. Liang et al, Effects of SodiumChloride Particles, Ozone, UV, and Relative Humidity on AtmosphericCorrosion of Silver, J. Electrochemical Society Vol. 157, No. 4, pagesC146-C156 (2010)). Both corrosion and tarnishing can be accelerated bythe presence of humidity and ozone in the atmosphere.

The Vickers hardness (HV) of silver is 100HV (electro-deposited), whichis low compared to the other end of the HV spectrum where diamond has avalue of 10,000HV. As a result of the relative softness of silvercompared to other materials, the handling of a silver coated optics forsystem assembly, or cleaning the optics which include mirrors, willoften result in damaging the silver surface. As a result a protectivelayer is needed to minimize damaging the silver surface. In order to beeffective the protective layer must be (1) sufficiently dense such thatno pathways are provided from the optic's surface to silver andinterface layers, (2) insoluble in alkali and acidic environments, (3)mechanically hard to provide scratch resistance, and (4) have either (a)only minimal absorption throughout the entire wavelength range ofinterest, 0.34 μm to 15 μm in the present case, or (b) no absorptionover the 0.34 μm to 15 μm wavelength range. Silicon nitride, Si₃N₄, wastested for its alkali diffusion properties, solubility in alkalisolution and for its mechanical hardness properties, and was found toprovide a very durable and chemically resistant coating. However, whilethis material was unfortunately found to have an absorption band atapproximately 9.1 μm, this disclosure shown that if the Si₃N₄ issufficiently thin it can be used. The exact thickness depends on thethroughput of the system for the band range. For some applications thisband is of no interest so the thickness is of limited consequence.

A single protective layer along with the ultra-thin interface layerreduces the stack reflection performance in the VIS range out into theSWIR bands, as observed in FIGS. 5A and 5B. Because differentapplications of the mirrors disclosed herein require the application ofthe tuning layers in order to optimize reflection in defined wavelengthregions, these tuning layers need to have characteristics similar tothose of the protective layer, but some minimal trade-offs can be madein the durability of these materials. To tune for the desiredreflectance bands the thickness of the tuning layer(s) will have to bevaried and a combination of low, medium and/or high index materials areused. The thickness of the tuning layer(s) is in the range of 75 nm to300 nm. The protective layer applied on top of the tuning layer(s) has athickness in the range of 60 nm to 200 nm.

Corning has developed thin film deposition processes, for example theprocess described in U.S. Pat. No. 7,242,843, which can be used forytterbium fluoride (YbF₃) and yttrium fluoride (YF₃), both of which arelow refractive index materials. When the process is used to coat highlyreflective silver mirrors the results indicate that the resultingmirrors are highly resistant to alkali solutions while also providingscratch resistance that meets the military specification moderateabrasion testing procedures; properties that will aid in protecting thesilver layer. The low refractive index materials were used incombination with high refractive index materials, for example, niobiumpentoxide (Nb₂O₅) and zinc sulfide (ZnS). FIG. 6A shows the reflectanceof (a) Ag (only), (b) Ag—Nb₂O₅ and (c) Ag—YbF₃ coatings over thewavelength range of 3 μm to 19 μm. In FIG. 6A the Ag (only) film has areflective of at least 98% over the wavelength range of the graph, 3 μmto 19 μm, except for a small decrease at approximately 18.6 μm, and issubstantially 99% reflective over the wavelength range of 3 μm toapproximately 17 μm. The Ag—Nb₂O₅ coating shows an Nb—O absorption withthe reflectivity dipping below 96% over the approximate wavelength rangeof 10 μm to 13 μm. The Ag—YbF₃ coating shows Yb—F absorption withreflectivity of greater than 98% in the wavelength range of 3 μm to 16.5μm. The Reflectance for all three coating was measured at AOI=45°.

Using YbF₃ as an exemplary low refractive index fluoride material, acoating combination of YbF₃—Nb₂O₅—YbF₃ was tuned for high reflectivityin the VIS range, 0.34 μm to 0.75 μm, and also in a MWIR-LWIR range of 3μm to 11.3 μm. FIG. 6B, Ag—Al₃O₃—YbF₃—Nb₂O₅—YbF₃ on 6061-Al having abarrier layer, shows that when measured in VIS-SWIR range of 0.4 μm to1.7 μm at an AOI of 45° the coating has a reflectivity of greater than96%, and when measured at an AOI 12° the reflectivity is substantially97%. FIG. 6C, Ag—Al₃O₃—YbF₃—Nb₂O₅—YbF₃ on 6061-Al having a barrierlayer, shows that in the MWIR-LWIR range of 4 μm to 15 μm, (a) at an ofAOI 12° the reflectivity is substantially 99% over the wavelength rangeand (b) at an AOI 45° the reflectivity was greater than 96% over thewavelength range and greater than 98.5% over substantially the entirerange of 3 μm to 15 μm. When oxygen is used during the deposition of theYbF₃ material the deposited layer becomes a ytterbium oxyfluoridematerials that is designate herein as Yb_(x)F_(y)O_(z), and thethickness of this layer is the in the same range as that for YbF₃.

The tuning layer and the protective layer can also be combined into asingle layer using a single material which is Yb_(x)F_(y)O_(z), Whenonly a single tuning/protective layer is used the thickness of the layeris in the range of 150 nm to 350 nm.

It was found that the combination YbF₃ (low refractive index) and ZnS(high refractive index) provides minimum absorption throughout thedesired wavelength range. FIG. 7 is a graph of reflectance versuswavelength of an Ag—Al₂O₃—YbF₃—ZnS—YbF₃ stack of coatings on a 6061-Alsubstrate having a barrier layer. The graph shows the LWIR absorption atapproximately 11 μm from the Al₂O₃ interface layer. The minimum Al₂O₃thickness required to achieve optimum adhesion still results inobservable absorption, unlike using an ultra-thin Ni layer as theinterface layer, which would results in less absorption.

Materials that found to be useful as protective layers are YbF₃, YF₃,and Si₃N₄. YbF₃ and YF₃ (low refractive index), GdF₃ (medium refractiveindex in VIS range), and ZnS and Bi₂O₃ (both high refractive index) arematerials that can be used for tuning and that have minimum absorptionin all bands including the LWIR. In addition, Si₃N₄ (medium refractiveindex), and Nb₂O₅, TiO₂ and Ta₂O₅ (all three high refractive index) canbe used as a tuning layer materials, but their LWIR absorption bandsneed to be considered in view of the application in which the mirrorwill be used.

Combining the Layers Together, and Testing for Durability and SpectralPerformance

Different combinations can be used to meet various militaryspecification environmental tests. The most difficult test tosuccessfully pass is the 24 hour salt fog test. The coating stacks usedfor FIGS. 6A and 6B were deposited on both fused silica substrates anddiamond turned 6061-Al substrates. The test results are shown in FIGS.8A and 8 D after >23 hours salt fog testing and 120 hours humiditytesting (respectively) at relative humidity, RH, of approximately 98% inaccord with the Mil-C-48497 specification. No measurable change wasdetected for the spectral performance for both tests, salt fog andhumidity. FIG. 8A indicates that the coating stack on fused silicasubstrate resulted in passing >23 hours of exposure to salt fog inaccordance per the Mil-C-48497 specification. In FIG. 8D, the identicalstack and barrier layer used for FIG. 8A was deposited on a 6061-Alsubstrate and the resulting mirror was exposed to 120 hours of humidityin accord with the same Mil-C-48497 specification.

FIGS. 8B and 8C illustrate the role of the barrier layer when a coatingstack is deposited on 6061-Al and the resulting mirror is exposed to thesalt fog environment. The stack break-down resistance is low for a thinbarrier layer and increases as the barrier layer becomes thicker. Themirror of FIG. 8B, which has a thick barrier layer, passed the salt fortest whereas the mirror of FIG. 8C, which has a thin barrier layer,developed defects which are pointed out by the arrows in the figure. Asindicated above, different materials can be for the barrier layer,including Si₃N₄, SiO₂, DLC, and CrN. These materials can be depositedusing different processes that have included IAD e-beam, low pressure DCmagnetron sputtering (U.S. Pat. No. 5,851,365, Corning Incorporated),CVD, sol-gel, and metal plating.

Process considerations for depositing the stack must be taken intoconsideration and these considerations are material and interfacedependent. Because film density and stoichiometry are critical, ion beambombardment is used during the deposition. Ion energies and densitiesmust be adjusted appropriately so as to densify, but not damage thefilm. Gas ratios of Ar, N₂, O₂ are adjusted to control the desiredstoichiometry, with the warning that O₂ should either be: (a) not beused during deposition of the Ag layer or the second interface layerafterwards, or (b) not used at the beginning of the deposition of thesecond interface layer, but added into the deposition process after avery thin, 3-5 nm second interface layer has been applied to the Aglayer, The objective is to have the second interface layer, for examplean oxide such as Al₂O₃, adhere to the silver layer while not exposingthe silver surface to excessive O₂ before the Al₂O₃ deposition, whilemaintaining substantially all of the Al₂O₃ at a stoichiometric or nearstoichiometric Al:O ratio. The following are some process criteria usedfor the stack.

-   -   1. If the Al₂O₃ is used as a barrier layer, then its initial        partial pressures of Ar—O₂ gases must be adjusted to create the        desired Al_(x)O_(y) stoichiometry needed to optimize adhesion at        the Al—Al_(x)O_(y) interface. The Al_(x)O_(y)—Ag interface        partial pressures are different than the Al—Al_(x)O_(y)        interface to achieve optimum adhesion so the process must be        adjusted towards the end of this barrier layer. The Al_(x)O_(y)        stoichiometry needed for optimum adhesion at either the Al or Ag        interfaces are discussed in the references above. Their partial        pressures or gas flow ratios will be dependent on deposition        rates, pumping speeds and deposition volumes.    -   2. Stoichiometry is also critical at the oxide-fluoride        interfaces to obtaining optimum adhesion. In the case of oxide        material, terminating the layer so that it is a stoichiometric        oxide is important; while the fluoride at the interface should        be an oxy-fluoride.    -   3. There are considerations to take into account when using IAD        during silver deposition; it is important not to exceed certain        ion energies and densities because it can result in trapping Ar        into the film. These defects can act as scattering centers which        will reduce reflectivity at the lower visible wavelength bands.    -   4. Bombardment energies and gas ratios should be adjusted to        obtain optimum film density. When densifying fluoride materials        one has to consider ion energies that will not dissociate the        fluorine atoms of the growing film. If this occurs the film will        become very unstable and spectral shifting will be observed.

Thus, in one aspect the disclosure is directed to a highly reflectivemirror having a for use in the wavelength range of 0.4 μm to 15 μm, themirror comprising a substrate, a barrier layer on the substrate, a firstinterface layer on top of the barrier layer, a reflective layer on topof the first interface layer, a second interface layer on top of thereflective layer, at least one tuning layer on top of the secondinterface layer and at least one protective layer on top of the tuninglayer, said mirror having a reflectivity of at least 96% over thewavelength ranges of 0.4 μm to 1.8 μm and 3 μm to 15 μm at an AOI 45°.The mirror has a reflectivity of at least 97% over the wavelength rangeof 0.4 μm to 1.8 μm and a reflectivity of greater than 98% over thewavelength range 0.4 μm to 1.8 μm at AOI 12°. The substrate that can beused in making the mirror can be selected from the groups consisting offused silica, fluorine doped fused silica and diamond turned aluminumalloys. In one embodiment, the substrate is 6061-Al alloy. In anotherembodiment, the substrate is fused silica. The barrier layer is selectedfrom the group consisting of Si₃N₄, SiO₂, TiAlN, TiAlSiN, TiO₂ and DLC.The first interface layer is selected from the group consisting ofAl₂O₃, TiO₂, Bi₂O₃ and ZnS, and the metallic materials Ni, Bi, Monel(Ni—Cu), Ti and Pt. The reflective layer is selected from the groupconsisting of zero valent Ag, Au, Al, Rh, Cu, Pt and Ni. The secondinterface layer is selected from the group consisting of SiO₂, Si₃N₄,Nb₂O₅, TiO₂, Ta₂O₅, Bi₂O₃, ZnS and Al₂O₃. The tuning layer is at leastone material selected from the group consisting of YbF₃, YF₃, GdF₃ andBi₂O₃. The protective layer is at least one material selected from thegroup consisting of YbF₃, YF₃ and Si₃N₄. An exemplary mirror accordingto the disclosure consists of, from substrate to the top protectivelayer, fused silica, Si₃N₄ or CrN, Al₂O₃, Ag, Al₂O₃, YbF₃, Nb₂O₅ andYbF₃.

The disclosure is also directed to a method for making a highlyreflective mirror, the method comprising the steps of:

providing a substrate selected from the group consisting of aluminumalloys, silica, fused silica, F-doped fused silica, magnesium alloys andtitanium alloys;

polishing the substrate to a roughness of less than 10 nm;

applying a barrier layer having a thickness in the range of 10 nm to 100μm to the surface of the substrate

applying a first interface layer having a thickness in the range of 0.2nm to 50 nm on top of the barrier layer, said thickness being dependenton the interface layer material;

applying a reflecting layer having a thickness in the range of 100 nm to300 nm on top of the adhesion layer;

applying a second interface layer of at least one selected material, thesecond interface layer having a thickness in the range of 0.2 nm to 50nm on top of the reflecting layer in top of the reflecting layer;

applying a tuning layer consisting comprising at least one selectedmaterial, the thickness of the tuning layer being dependent on the atleast one selected material(s); and

applying at least protective layer on top of the tuning layer to therebyform a highly reflective mirror having a reflectance of at least 96%over the wavelength range of 0.4 μm to 15 μm.

In the foregoing method the barrier layer material is selected from thegroup consisting of Si₃N₄, SiO₂, TiAlN, TiAlSiN, TiO₂, DLC, Al and CrN;the first interface layer material is selected from the group consistingof Al₂O₃, TiO₂, Bi₂O₃ and ZnS, and the metallic materials Ni, Monel(Ni—Cu), Ti and Pt; the reflective layer is a material selected from thegroup consisting of Ag, Au, Al, Rh, Cu, Pt and Ni; the second interfacelayer is at least one material selected from the group consisting ofSiO₂, Si₃N₄, Nb₂O₅, TiO₂, Ta₂O₅ and Al₂O₃; the tuning layer is at leastone material selected from the group consisting of YbF₃, YF₃, GdF₃ andBi₂O₃; and the at least one protective layer is selected from the groupconsisting of YbF₃, YF₃, Si₃N₄. In an embodiment the reflective layer issilver. In an embodiment the second interface layer comprisesSi₃N₄—SiO₂—Si₃N₄.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

We claim:
 1. A highly reflective mirror for use in the wavelength rangeof 0.4 μm to 15 μm, the mirror comprising: a substrate, the substratecomprising magnesium or titanium; a barrier layer on the substrate, thebarrier layer comprising Si₃N₄ or CrN; a first interface layer on top ofthe barrier layer, the first interface layer comprising Al₂O₃; areflective layer on top of the first interface layer, the reflectivelayer comprising Ag; a second interface layer on top of the reflectivelayer, the second interface layer comprising Al₂O₃; at least one tuninglayer on top of the second interface layer, the at least one tuninglayer comprising at least one from the group consisting of (i) YbF₃ plusNb₂O₅ and (ii) Yb_(x)F_(y)O_(z); and at least one protective layer ontop of the tuning layer, the at least one protective layer comprisingYbF₃; said mirror have a reflectivity of at least 96% over thewavelength ranges of 0.4 μm to 1.8 μm and 3 μm to 15 μm at an AOI 45°.2. The mirror according to claim 1, wherein the mirror has areflectivity of at least 97% over the wavelength range of 0.4 μm to 1.8μm and a reflectivity of greater than 98% over the wavelength range 0.4μm to 1.8 μm at AOI 12°.
 3. The mirror according to claim 1, wherein thesubstrate comprises a magnesium alloy or a titanium alloy.
 4. The mirroraccording to claim 1, wherein the barrier layer material has a thicknessin the range of 10 nm to 50 μm.
 5. The mirror according to claim 1,wherein the first interface layer material has a thickness in the rangeof 0.2 nm to 25 nm.
 6. The mirror according to claim 1, wherein thereflective layer has a thickness in the range of 75 nm to 350 nm.
 7. Themirror according to claim 6, wherein the reflective layer has athickness in the range of 80 nm to 150 nm.
 8. The mirror according toclaim 1, wherein the thickness of the second interface layer is in therange of 5 nm to 20 nm.
 9. The mirror according to claim 8, wherein thethickness of the second interface layer is in the range of 8 nm to 15nm.
 10. The mirror according to claim 8, wherein the thickness of thesecond interface layer is in the range of 8 nm to 12 nm.
 11. The mirroraccording to claim 1, wherein the thickness of the at least one tuninglayer is in the range of 75 nm to 300 nm.
 12. The mirror according toclaim 1, wherein the protective layer has a thickness in the range of 60nm to 200 nm.